Various processes and systems for the manufacture of acrylonitrile and methacrylonitrile are known; see for example, U.S. Pat. No. 6,107,509. As noted in U.S. Pat. No. 6,107,509, conventional processes typically involve recovery and purification of acrylonitrile/methacrylonitrile produced by the direct reaction of a hydrocarbon selected from the group consisting of propane, propylene or isobutylene, ammonia and oxygen in the presence of a catalyst has been accomplished by transporting the reactor effluent containing acrylonitrile/methacrylonitrile to a first column (quench) where the reactor effluent is cooled with a first aqueous stream, transporting the cooled effluent containing acrylonitrile/methacrylonitrile into a second column (absorber) where the cooled effluent is contacted with a second aqueous stream to absorb the acrylonitrile/methacrylonitrile into the second aqueous stream, transporting the second aqueous stream containing the acrylonitrile/methacrylonitrile from the second column to a first distillation column (recovery column) for separation of the crude acrylonitrile/methacrylonitrile from the second aqueous stream, and transporting the separated crude acrylonitrile/methacrylonitrile to a second distillation column (heads column) to remove at least some impurities from the crude acrylonitrile/ methacrylonitrile, and transporting the partially purified acrylonitrile/methacrylonitrile to a third distillation column (product column) to obtain product acrylonitrile/methacrylonitrile. U.S. Pat. Nos. 4,234,510; 3,885,928; 3,352,764; 3,198,750 and 3,044,966 are illustrative of typical recovery and purification processes for acrylonitrile and methacrylonitrile.
It is important for acrylonitrile product to meet specifications, including the maximum amount of certain chemical impurities, and predetermined amounts of water and polymerization inhibitor. Control of the water and polymerization content of acrylonitrile product is important to the stability of the acrylonitrile product. The amount of water and monomethyl ether hydroquinone or methyl ether of hydroquinone (hereinafter “MEHQ”) polymerization inhibitor in acrylonitrile product typically must be maintained within specified levels or ranges. The MEHQ polymerization inhibitor requires a certain amount of water in order to function and inhibit undesirable polymerization of acrylonitrile product. Specifications, such as U.S. and international shipping specifications, typically require a minimum amount of water and inhibitor in the acrylonitrile product to be shipped because the acrylonitrile product without a certain amount of water and inhibitor is highly reactive. Specifications typically have a maximum amount of water and inhibitor content as well because excessive water and inhibitor will make desired subsequent reaction of acrylonitrile slow and difficult, e.g., at a customer plant designed to polymerize acrylonitrile. Specifications for the acrylonitrile product typically have maximums on certain impurities, such as hydrogen cyanide, acetonitrile, and acetone.
A conventional process has the following steps. First, acrylonitrile product from the final distillation column is accumulated in one of two or more product rundown tanks. These tanks are equipped with mixers, and have provision for addition of water and MEHQ polymerization inhibitor. Next, a sample is taken of the acrylonitrile product that has accumulated in the rundown tank. Next, this sample is analyzed in the laboratory for water content, MEHQ content, and other product specifications. If the water in the acrylonitrile is below the specified amount, a calculation is made on the amount of water that must be added in order to meet the specification. This amount of water is then added manually to the rundown tank. Likewise, if the MEHQ in the acrylonitrile is below the specified amount, a calculation is made on the amount of MEHQ solution that must be added in order to meet the specification. This amount of MEHQ solution is then added manually to the rundown tank. The rundown tank is then mixed for a specified period of time.
After mixing, the acrylonitrile in the rundown tank is re-sampled. This second sample is analyzed in the laboratory for water content, MEHQ content, and other product specifications. If analysis of the sample of acrylonitrile in the rundown tank indicates that the contents meet the specifications, the acrylonitrile is then transferred from the rundown tank to one of the final product storage tanks. The above conventional process requires multiple rundown tanks so that acrylonitrile can be produced continuously while the steps listed above are completed. The conventional process includes the need to make final adjustments to water and MEHQ content in at least one rundown tank, and multiple steps that require time to complete before acrylonitrile product is brought within specification and can be released to final product storage. The conventional process requires plant and laboratory time and labor for the drawing of samples in the plant, transferring the drawn samples to the laboratory, analyzing the samples in the laboratory, relaying the laboratory analysis to plant operators, determining of action based on the laboratory analysis, and the taking of action in the plant in an attempt to bring the product in the at least one rundown tank within specification. In some instances, multiple steps may not be accomplished in sufficient time for taking of corrective action at the rundown tank. In such instances, an amount of product in the rundown tank must then be re-run through equipment upstream of the rundown in an attempt to bring the product to within specification. The conventional process typically requires at least three rundown tanks, wherein there is filling of a first tank with acrylonitrile product, analyzing of acrylonitrile product in a second tank that was previously filled, with no flow of acrylonitrile product entering or exiting from the second tank during the step analyzing, and pumping acrylonitrile product out of third tank after filling and analyzing of acrylonitrile product in the third tank was completed.